Where are all the missing links?

As I keep saying in the Land Mammal to Whale transition: fossils Part II thread, this fossilization issue needs to be settled before we can productively discuss much else. You're applying false logic to conclude that finding thousands of fossils of some species means that fossilization is not rare. In order to conclude that you would have to know how many individuals of the species lived over geological time.It works like this. Say you find 1,000 fossilized individuals of species X. Is fossilization of species X common? Well, you would have to know how many individuals of species X ever lived. If only 1,000 individuals ever lived, then fossilization was very common, since 100% of the species was fossilized.But what if the number of individuals that ever lived was 10 billion? That would yield a fossilization rate of .00001%, which isn't very common at all.It also works like this. Again lets say you've found just 1 fossilized individual of a species. Does this mean fossilization is rare? It depends. If only 1 individual of this species ever lived, then fossilization is common. But if 10 billion lived, then fossilization is rare.If you're going to make your determination of the likelihood of fossilization based solely upon the number of fossils found, then you also have to know the number of individuals of each species that ever lived.Complicating such determinations are the precise same issues I've mentioned several times already:

• Fossils can be preserved in layers that subsequently erode away and disappear forever.
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• Fossils can be preserved in layers that are not accessible, being buried under layers of geological strata.
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• Fossils preserved in sea floor can subduct under continents.
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• On land, fossils are usually only preserved in lowland regions subject to net deposition, such as shore regions and swamps.
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• Evolutionary change is more likely in small populations than large, and naturally the probability of fossilization is smaller with smaller populations.

You might remember that some time ago there was a crisis in Yellowstone, perhaps it was a hard winter, that caused the starvation death of thousands of Elk, and since the herd is too large some die every winter anyway, yet you can't visit Yellowstone today and find a single Elk skeleton. How many times have you walked through the woods and found animal skeletons? On the savannah in Africa you can watch those documentaries showing predation of elk or zebra, but the remains disappear in very short order. As the Bible says, ashes to ashes, dust to dust, and the ultimate fate of almost all creatures is dust, not fossilization.--Percy

Let me try explaining this by way of example. Let's say we have two similar species A and C, and that from stratigraphy and radiometric dating it is obvious that A preceded C, and that therefore C must have evolved from A or from some similar cousin. But A and C are sufficiently different that it seems reasonable to think that there should have existed an intermediate species B. But species B is nowhere to be found. How could that be?There are many possibilities, but I'm just going to mention a few:

• Species A was decimated by some catastophe, and the survivors evolved over a relatively short time period into species B, which was marginally successful, and then into species C in a small geographical region leaving some fossil remains behind, but we haven't discovered them yet because this region is currently buried under a mile of geological strata. Species C was very successful and repopulated the area formerly occupied by species A.
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• Now imagine the same scenario, except this time the survivors of species A are in small geographical region off a continental shelf. The same thing happens as before, but this time instead of the region being buried deep beneath geological strata, the region subducts under the continent and is gone forever, including all fossils of species B.
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• Now imagine the same scenario, except this time species B doesn't happen to occupy a region where fossilization is likely. No fossils of species B exist or ever existed.
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• Now a different scenario. In this one it is simply that the intuition of paleontologists are wrong in this case. Because of environmental pressures, species A or a cousin evolved in gradual (but short geologically) stages into species C, never pausing for any extended period at a particular stage of development. This happened quickly enough that too few individuals were ever fossilized to make discovery likely.
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• Just for completeness and not because I think it a realistic possibility, species A somehow became species B all at once in a sudden jump.
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• And lastly a scenario I consider completely unscientific, species A went extinct and species B was created by some intelligent agent.

Given this information, what makes you think that you can arrive at any definite conclusions about what happened to species B, or whether there even was a species B?Moving on: No, because large populations swamp small changes. It's like if you shout at a chamber music recital then you'll easily be heard, but if you shout at a rock concert then only the people nearby will hear and your voice will be drowned out after any distance. Or if you throw a large rock into a small pond it has an enormous effect, but if you throw the same rock into the ocean it has a negligible effect.In the same way, a mutation or rare confluence of alleles can have a significant impact on a small population. But the same change will likely get swamped out in a larger population and disappear.Another factor at work is that a very large population implies a very stable and conosistent environment across a large region, and the evolutionary pressures for change in such a situation are small and probably cyclical, in that the pressures first shift a little one way and then the other way from year to year, but pretty much stay fairly near a mean. Any significant environmental change, even if it affects the entire region, is not going to affect the entire region in the same way, and so environmental change has the tendency to split large populations into many smaller populations, each experiencing different environmental pressures.There *are* some fossilization patterns I can think of that would correlate with larger population size, such as appearance across a broad geographic region and/or through many geologic strata of time. There must be others, too. You're not specific enough for me to follow your meaning, but I don't see how you can conclude anything about numbers of species or numbers of individuals just from the fact that evolution is actually a continuum rather than a sequence of discrete species-sized steps. Can you explain how you arrived at this conclusion?AbE: Later it occurred to me what you mean. You mean that if a species blends smoothly into succeeding species, how can branching ever occur? How can one species ever become two species or more?There must be many different ways this can happen, but the one that gets mentioned most often in these discussions is that a population's range becomes somehow divided so that one large population becomes two smaller populations. A river changes course, a migration route becomes temporarily blocked, gradual elevation changes occur, marine incursions happen, predators migrate into a central region, weather patterns make a central part of the population's range that use to connect them untenable, a desert once narrow enough to easily traverse becomes too large, competition from other creatures at one end of the range differs from creatures at the other end, and so on. There are literally a huge number of things that can divide a population.Once a population is divided and is no longer sharing the same gene pool, the differing environmental pressures cause the two populations to evolve in different directions, and the gene pools, both in terms of mutations and allele frequency, also become more different.It is also amazing to realize that even if the separated populations are subjected to identical environmental pressures (as much as is possible), they'll still evolve in different directions. That's because which mutations occur and which alleles get combined is largely random (except for unsuitable individuals which die and remove themselves from the gene pool - natural selection is not random). Mutations are of course random, and which mates choose each other is largely random, and mating is of course fairly random at the sperm/egg level. The genetic evolutionary engine of change will often arrive at unique solutions to identical problems, because it is working from scratch every time with no knowledge of how other species might have solved similar problems (unless the species was an ancestor, in which case the solution might still be lurking around somewhere in the DNA just waiting for some genetic change or circumstance to turn it on).--PercyThis message has been edited by Percy, 08-16-2005 04:55 AM

Hi Randman,I know you haven't had a chance to respond to Message 111 yet, but I thought it would be helpful to add a little bit about how one species transitions in continuous fashion into another. I'll just focus on sexual species.Every reproductive act is a tiny evolutionary step. We are all different from our parents, who are different from our grandparents, who are different from our great grandparents, and so forth. The genetic code of babies is never identical to either parent. Each new human being is a unique individual genetically (ignoring the special case of identical twins, of course).This tiny change comes from two sources. One is unique allele combinations. An allele is one type of a gene. To explain this a little better, let's say there's an organism whose eye color is determined by a single gene. The different eye colors are the alleles of the gene. There would be a gene allele for blue, another for brown, another for green, and so forth. Each parent contributes an allele for eye color. If both alleles are the same then the organism's eye color will be whichever allele that is. If the two alleles are different then the dominant allele determines eye color. For example, in humans brown is generally dominant over blue, and so when one parent contributes a brown allele and the other contributes a blue, the child's eye color will be brown (I think eye color in humans is determined by more than one gene, and so it may not be as simple as I'm protraying here, but I'm only trying to get the principle across).To continue with humans as an example, since humans have somewhere around 30,000 genes, if we assumed that each gene had only two alleles then the number of possible allele combinations is around 10⁹⁰⁰⁰. Since only around 10¹⁰ humans have ever lived, there's only one chance in 10⁹⁰⁰ of an allele combination appearing that is not unique. 10⁹⁰⁰ is a very large number. It's a 1 with 900 zeros after it. Or you could say that it's a billion billion billion billion billion...(keep this up until you've said billion 100 times). The likelihood that each newborn baby is genetically unique is extremely close to 1. In other words, the probability of any allele combination occuring more than once is very small, and keep in mind that many genes have more than one allele, which increases the odds of uniqueness even more. The odds are reduced somewhat because some allele combinations are hostile to fetus viability, and the baby is never born, often shed so early that the women is never even aware an egg was fertilized.But I mentioned another factor that contributes to the tiny change associated with each reproductive event, and that's mutation. Mutations are caused by imperfect copying of the genetic material, DNA. The estimates I've seen say that on average around 10 mutations occur in the DNA of every new human being. This raises even further the likelihood that each newborn individual will be genetically unique. Most mutations are tiny, only a single base pair missing or out of order or replaced by the wrong base pair or matched up with the wrong base. But some mutations can be larger, such as a gene being inverted or moved to a different part of the chromosome or even to a different chromosome. There can even be extra or missing chromosomes. The more significant the mutation, the more likely it is to be fatal.So we've established that every reproductive act creates a unique individual both in terms of allele combinations, and due to the addition of some usually harmless mutations. So if each child is different from the parents, and if this is true of the child's eventual children, and their children, and their children after them, and so on into the distant future, then it can be seen that change can only accumulate. There are forces acting to harness this change, but I'll discuss those later.As this change accumulates, at which point does one species become another? That turns out to be a difficult question to answer, and is probably best explained using the example of ring species.The most famous example of a ring species is the herring gull. You can find information about it from all over the Internet, but here's a short excerpt from this one: Ring Species. (If you delve more deeply into herring gulls you'll find it isn't quite this simple or neat, but there's no need introduce extraneous details for this discussion.)The point of introducing ring species is to help explain that the more genetically similar two species are, the more likely they are to be able to interbreed. There is no miraculous sudden cut-off point. Consider two fairly similar populations of the same species that are gradually drifting apart genetically. It isn't like one year they can interbreed and the next year they can't. What actually happens is that as they become more and more genetically different, fewer and fewer individuals of those populations are able to interbreed, until eventually it is so few that they should be considered different species.So with the herring gull you can ask the question, at which point as you circle the world from Britain to eastern America to Alaska to Siberia and back to Britain does the herring gull become a different species than the Lesser Black-Backed gull. There's no unamibiguous answer to this question. When two populations interbreed then it is said that there is a gene flow between them, and As Don Linday accurately says in the link I provided, "Two species are the same if there is 'significant' gene flow between them. But there is no sharp dividing line between 'significant' and 'insignificant'."I earlier mentioned that there are forces that harness or rein in the degree of change in a species. One is natural selection. For a well-adapted species in a stable habitat, any significant change may be maladaptive, and such individuals are less likely to survive to reproduce, or are less likely to produce as many offspring as the average.Another force harnessing in change is the size of the population. This has been mentioned more than several times by more than one person here. A species with a large population (human beings come to mind again) will have a large range and hence experience a variety of environmental conditions. At the extremes of the range the evolutionary forces for change are greatest, but genetic change from these regions all flow into the large central population and are diluted.It also takes much longer for change to propagate through a large population. Take a population of animals with a very broad range, perhaps the bison of the American plains, but before, say, 1800, which would predate when the herds began to be decimated. An individual with a beneficial mutation or allele combination that lived at one end of the range would produce offspring who possessed this genetic trait, and the question concerns how far those offspring might travel during their reproductive lifetimes. How far that range is governs how fast the trait can spread through the population. Reproductive rates are also a significant consideration. Creatures that stay relatively close to home during their lifetime and which do not reproduce frequently can spread their traits throughout the population's range only very slowly. Those who have the opposite qualities, those who range far and reproduce often, can spread their traits quickly through a very broad range.I've hope I've shown that evolutionary change is continuous, and that the division of populations into distinct species is not an unambiguous task. I hope I've additionally shown that genetic changes can propagate much more quickly throughout a small population than a large.--PercyThis message has been edited by Percy, 08-19-2005 09:23 AM

Hi Evopeach,I thought your quotes about other scientists rejecting the view that there was something cognitively similar to humans regarding grammatical speech going on in gorillas and chimpanzee minds were pretty good rebuttal, but the to me the true issue isn't the degree to which other apes approach human cognitive skills, but whether there seems to be a transition among extant ape species from lesser to greater cognitive skills as they become genetically closer to humans. It seems that way to me. Doesn't it to you? The above appears to be off topic for a thread about missing links.One question that comes up on the issue of transitionals is at what level of fineness does a transition become simply a variation. To use an musical analogy, say you're seeking the transitional notes between middle C and the C one octave above. You find F, and then someone asks are there any notes intermediate between middle C and F. So you find D and someone asks if there are notes intermediate between middle C and D. So you find C#, and someone asks if you can find notes intermediate between middle C and C#. And you reply that there are no notes between middle C and C#. You explain that as the frequency of middle C is increased it gradually just becomes a more and more off-tune C or a less and less off-tune C#.But of course this is just an artifact of the way we hear and of our western music. There *are* musical cultures which use quarter tones (e.g., halfway between C and C#). There's nothing sacred about our division of frequencies into notes.So the question is, when is a transition filled with enough intermediates to no longer require additional transitionals? How do we tell when we're done?--Percy

I don't think Rahvin was saying this. Even though our closest living relative, the chimpanzee, is somewhere around 97% similar to humans genetically, with at least 30,000 genes this represents a difference in a potentially large number of genes (depends how the differences are distributed, which I'm not familiar with). A fair number of those genetic differences must be brain related, so I don't think anyone could reasonably claim, and to my knowledge no one here is claiming, that the difference in human and, for example, chimpanzee brain is due to only "a few minor genetic mutations".I think Rahvin is arguing that many of the structures of the human brain are shared by other apes, and that we just have more and better of them. I think your rebuttal needs to focus on those brain structures which are completely novel in human beings in that they are not shared by other apes.--Percy